1. Field of the Invention
The present invention relates generally to binder compositions for use in molding ceramic particulate compositions, and more particularly, to a binder system for forming arc tubes for ceramic arc discharge lamps.
2. Discussion of the Art
Discharge lamps produce light by ionizing a fill material, such as a mixture of metal halide and mercury, with an arc passing between the two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber, which maintains the pressure of the energized fill material and allows the emitted light to pass through. The fill material, also known as a xe2x80x9cdose,xe2x80x9d emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, including color temperatures, color rendering, and luminous efficiency.
Arc tube chambers composed of fused silica xe2x80x9cquartzxe2x80x9d are readily formed. However, the lifetime of such lamps is often limited by the loss of the metal portion of the metal halide fill (typically sodium) during lamp operation. Sodium ions diffuse through, or react with, the fused silica arc tube, resulting in a corresponding build-up of free halogen in the arc tube. Quartz arc tubes are relatively porous to sodium ions. During lamp operation, sodium passes from the hot plasma and through the arc tube wall to the cooler region between the arc tube and the outer jacket or envelope. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission. The light output consequently diminishes and the color shifts from white toward blue. The arc becomes constricted and, particularly in a horizontally operated lamp, may bow against the arc tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained, ending the life of the lamp.
Ceramic discharge lamp chambers were developed to operate at higher temperatures than quartz, i.e., above 950xc2x0 C., for improved color temperature, color rendering, and luminous efficacies, while significantly reducing reaction with the fill material. U.S. Pat. Nos. 5,424,609; 5,698,984; and 5,751,111 provide examples of such arc tubes. While quartz arc tubes are limited to operating temperatures of around 950xc2x0 C. to 1000xc2x0 C., due to reaction of the halide fill with a gas, ceramic alumina arc tubes are able capable of withstanding operating temperatures of 1700xc2x0 C. to 1900xc2x0 C. The higher operating temperatures provide better color rendering and high lamp efficiencies. Ceramic arc tubes are less porous to sodium ions than quartz tubes and thus retain the metal within the lamp. Various techniques are available for fabricating the arc tubes, including casting, forging, machining, and various powder processing methods, such as powder injection molding (PIM). In powder processing, a ceramic powder, such as alumina, is supported by a carrier fluid, such as a water-based solution, mixture of organic liquids, or molten polymers. The mixture can be made to emulate a liquid, a plastic, or a rigid solid, by controlling the type and amount of carrier and the ambient conditions (e.g., temperature).
For forming relatively complex parts, such as arc tubes, it is common to mold sub-components and join them together after molding. The parts are extruded or dye pressed from a ceramic powder mixed with a carrier fluid comprising an organic binder. European Patent Application No. 0587238 A1, for example, discloses a ceramic discharge tube of translucent aluminum oxide. The result of the shaping process is a xe2x80x9cgreenxe2x80x9d (i.e., unfired) powdered compact that is a solid, but has an internal structure that consists of discreet powder particles held together by the action of the binder. The powder compact is converted to a dense solid through thermal processing or xe2x80x9csintering,xe2x80x9d which burns out or pyrolizes the organic phase and densifies or sinters the inorganic powder.
To form cohesive gas tight joints between the components, a joint compound, such as a polymer may be used, which seals the component parts together on fusing. By working in the green state, the joining operation can form a bond through action on the organic binder, rather than directly on the metal or ceramic. Alternatively, the component parts can be fused directly together. The joining operation is chosen to be compatible with subsequent thermal processing and not to interfere with densification
In some cases the binder consists of a mixture of two or more materials. During binder removal, the process conditions are controlled, such that one component of the binder is preferentially removed while the other remains in the compact. At this intermediate stage, the powder compact develops porosity and becomes functionally equivalent to a compact of the type produced with a solvent-based system. Binder removal can be accomplished in several ways, including, for example, acid etching, solvent leaching, or thermal extraction. The removal process removes most, for example, 95% of the binder, prior to sintering, creating what is sometimes called a xe2x80x9cbrownxe2x80x9d body. The residual binder in the brown body provides sufficient strength to the body to allow handling. The residual binder is eliminated after joining the component parts, during the initial stages of sintering.
The green or brown arc tube bodies are fragile, since the microstructure of the tube is not developed until sintering occurs. The small diameter legs of the arc tube end plugs, which carry the electrodes into the arc tube body, are particularly susceptible to breakage. Without extremely careful handling, a significant portion of the green or brown arc tubes can be wasted through breakage. Sometimes, the parts stick to the mold and break. The molds must then be opened and cleaned to remove broken parts. The parts can also be damaged during post-molding processes, such as machining.
Attempts have been made to improve the green strength of ceramic through modifications of the binder system. U.S. Pat. No. 5,332,537, for example, discloses chemically cross-linking a binder to increase the green strength. Such chemical cross-linking binders are typically superior in strength properties to paraffin-based binder systems. However, the chemically cross-linked systems cannot be recycled once the cross-linking process is complete. Moreover, chemical cross-linking can occur prematurely, i.e., during feedstock preparation. Furthermore, paraffin-based systems tend to have relatively low viscosity, which reduces contaminants picked up during processing. These contaminants can be detrimental to the optical properties of a ceramic arctube.
U.S. Pat. No. 4,734,237 discloses gelation in water based systems which provides physical cross-linking, to improve green strength.
U.S. Pat. No. 4,571,414 discloses a binder formulation comprising a blend of poly(ethylene-co-vinyl acetate) and an organic acid. The formulation has a relatively high viscosity. In order to create a flowable material, the binder is heated to an elevated temperature. U.S. Pat. No. 5,254,613 discloses a binder formulation comprising a blend of poly (ethylene-co-vinyl acetate) and two wax components with freezing points above 80xc2x0 C.
The present invention provides a new and improved binder system and method of use, which overcomes the above-referenced problems, and others.
In an exemplary embodiment of the present invention an injection moldable binder system for a sinterable powder is provided. The binder system includes a hydrocarbon and a copolymer which co-crystallizes with the hydrocarbon when the binder system is cooled.
In another exemplary embodiment, an injection moldable composition is provided. The composition includes a sinterable powder and a binder system. The binder system includes a hydrocarbon and a copolymer which co-crystallizes with the hydrocarbon when the binder system is cooled.
In another exemplary embodiment, a method of forming a ceramic body is provided. The method includes combining a binder system with a sinterable powder at a temperature above the melting point of the binder system. The binder system comprises a hydrocarbon and a copolymer. The binder system and the sinterable powder are formed into a shaped body. The hydrocarbon and the copolymer are selected such that segments of the copolymer co-crystallize with the hydrocarbon as the binder cools. The method further includes removing at least a major portion of the binder from the shaped body, and heating the shaped body at a sufficient temperature and for a sufficient time to form the ceramic body.
One advantage of the present invention is in improved green strength of molded ceramic components.
Another advantage of the present invention is that it enables unused binder to be recycled.
Another advantage of the present invention is that the binder may be processed at temperatures well below 100xc2x0 C., enabling water heating to be used.
Another advantage is the current binder has a relatively low viscosity at temperatures below 100xc2x0 C., which minimizes contamination due to abrasive wear and pick up of abraded particles.
Another advantage of the present invention is that the binder may be removed from a green ceramic body without producing significant defects, such as voids or cracks.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.